The basic principle of OCT is based on white light interferometry. This method compares the propagation time of a signal using an interferometer, usually a Michelson interferometer. In this case an optical path with known optical wavelength, the reference path of the interferometer, is used as the reference for the measuring path. The interference of the signals from both paths produces a pattern, as a result of optical cross correlation, from which pattern the relative optical wavelength—an individual depth signal—can be read. In the one-dimensional grid method the beam is transversely guided in one or two directions, whereby a two-dimensional scan or a three-dimensional tomogram may be taken. The outstanding property of the OCT lies in the decoupling of the transversal resolution from the longitudinal resolution. In contrast thereto both the axial resolution—depth-wise—and the transversal—lateral—resolution depend on the focusing of the light beam in conventional light-optical microscopy.
The fields of application are primarily in medicine, in particular in ophthalmology, and in early cancer diagnosis, for skin examination or in the field of examination of vessels which is considered in particular here. Reflections on boundaries of materials with different refractive indices (membranes, cellular layers, organ boundaries) are measured in this case and thus a two- or three-dimensional image is reconstructed.
The use of OCT is limited by the penetration depth of electromagnetic radiation into the object being examined and by the bandwidth. Since 1996 sophisticated broadband [lasers] have enabled the development of UHR-OCT (Ultra High Resolution OCT) which has advanced the resolution from a few tens of micrometers (μm) to fractions of micrometers. Subcellular structures in human cancer cells can thus be displayed.
In the field of vessel examination OCT is used, as is described for example in WO 97/32182 A1, to generate images from the insides of the vessels using image-producing intravascular catheters. OCT is particularly suitable for example for qualitative plaque assessment. For this purpose OCT systems operate in a light wave range of approx. 1,300 nm. The light is emitted into the vessel wall from a catheter introduced into a vessel and the reflection from the vessel wall is registered in a depth-resolved manner, as described above, by means of interferometry. By translating the irradiated light beam information from various adjacent locations in the vessel wall can be obtained and compiled into a 2D image. The catheter can also be moved in the longitudinal axis of the vessel during image acquisition in order to sequentially display portions of the vessel that are located one behind the other.
The reflections of the various vessel wall layers carry the relevant image information and must be detected and displayed by the OCT device. The catheter itself has an internal structure which produces reflections, so the OCT system displays the catheter used in the image. OCT and the interferometry used for this purpose involve minimal differences in length which have to be detected. As, during catheter production, the manufacturing tolerances of the catheter length far exceed the differences in length that are to be measured, the OCT system must be re-calibrated for each new OCT catheter which is used as disposable material.
It is known here to make use of the autoreflections from the catheter itself. As the manufacturing tolerance of the diameter of the OCT catheter, which is visible in the center of the OCT image, is negligible compared with the differences in length that are to be measured, markings are usually displayed on the screen of the OCT system for calibration purposes, the spacing of which markings from each other corresponds to the known diameter of the OCT catheter. During calibration the user manually adjusts the length of the measuring path until the marking and the displayed reflection of the catheter match.
If the OCT catheter is advanced or withdrawn the optical fiber, which is part of the measuring path, in the catheter is compressed or elongated by a few micrometers. The changed length of the OCT catheter, or its core, changes the calibration, so length measurements in photographs which are caused by a movement of the catheter contain substantial errors and thus cannot be utilized for the examination. Manual recalibration is not possible as the catheter moves very quickly.